1. Field of the Invention
The present invention relates to a key sheet for a pushbutton switch for use in the operating portion of an apparatus, such as a mobile phone, a PDA, a car navigation apparatus, or a car audio apparatus, and more specifically, to a key sheet suitable for a case in which a plurality of keytops are exposed for use through an operational opening of a casing having no partition frame for separating from each other the plurality of keytops exposed from the casing of an apparatus.
2. Description of the Related Art
For example, a pushbutton switch is known in which, as in the case of a mobile phone 101 shown in FIG. 18, due to requirements in terms of miniaturization, artistic design property, etc. of the operating portion, a plurality of keytops 103 of a key sheet 102 that are arranged close to each other are exposed through an operational opening 101b with no partition frame formed in a casing 101a. As shown in FIG. 19, this key sheet 102 equipped with the keytops 103 consists of a base sheet 104 formed of silicone rubber to which a plurality of (seventeen in total in this case) keytops 103 are firmly attached. Specifically, this key sheet is composed of a large keytop 103a situated in the upper middle portion thereof and used for directional inputs in the vertical and horizontal directions, four small keytops 103b situated on the right-hand and left-hand sides thereof, and twelve medium-sized keytops 103c situated below them. The gaps between the adjacent keytops 103a, 103b, and 103c are very narrow, the keytops being arranged at a pitch, for example, of as small as approximately 0.15 mm to 0.2 mm. The gap between the keytops and the operational opening 101b is also very narrow, its dimension being approximately the same as mentioned above.
As shown in FIG. 20, this key sheet 102 is mounted to the mobile phone 101, with the base sheet 104 being sandwiched between and in press contact with the back surface of the casing 101a and a circuit board 101d. Usually, the base sheet 104 is formed of soft silicone rubber, so that when, for example, as shown in FIG. 21, during use of the mobile phone 101, it is set upright or when, as shown in FIG. 22, it is laid face down, the base sheet 104 may expand in its entirety under the weight of the keytops 103 to be thereby distorted. This may lead to positional deviation between pushers 104a of the base sheet 104 and contact switches 101e provided on the circuit board 101d, which is composed of metal disc springs and a contact circuit, resulting in a malfunction, making it impossible to effect input even when the keytops 103 are depressed.
To eliminate this problem, the present applicant has proposed a key sheet 111 as shown in FIG. 23 (JP 2003-114833 A). This key sheet 111 has a base sheet 112 equipped with a thin, hard resin plate 113. Further, the hard resin plate 113 has, at the positions where the keytops 103 are to be provided, rectangular through-holes 114 formed by lattice-like frame portions 113a, and pedestal portions 115 formed of a rubber-like elastic material and formed so as to close the through-holes 114. In this key sheet 111, the frame portions 113a of the hard resin plate 113 serve as frame-like support portions effecting floating support of the pedestal portions 115 so as to allow their displacement, so that it is possible to restrain generation of an overall distortion of the base sheet 104 as shown in FIG. 21 or 22.
Incidentally, it is often desirable for the mobile phone 101, whose casing 101a has no partition frame, to be of a thin type. That is, in the mounting structure as shown in FIG. 24, in which the key sheet 111 is mounted to the casing 101a, there is a demand for a further reduction in a distance d1 between the circuit board 101d and the casing 101a. If, to meet this demand, the pedestal portions 115 are made thinner to be flush with the surface of the base sheet 112, as shown, for example, in FIG. 25, the keytops 103 will be caught by the hard resin plate 113, making it impossible to fully depress the keytops 103 so as to reliably effect contact input.